1. Field of the Invention
The present invention relates to an image display device, and particularly to an image display device provided with a circuit employing a thin film transistor capable of high-speed operation.
2. Description of Prior Art
Polycrystalline silicon thin film transistors (hereinafter also called polycrystalline silicon TFTs) have been developed as active elements forming pixels or pixel-driving circuits for active-matrix type liquid crystal display device (liquid crystal displays), organic light-emitting display devices (organic EL displays), and image sensors. Polycrystalline silicon TFTs have an advantage of greater driving capability over other driving circuit elements and peripheral circuits formed by polycrystalline silicon TFTs can be mounted on the same substrate mounting the pixels thereon.
In a case in which polycrystalline silicon TFTs are used for large-sized liquid crystal display devices such as TV receivers and large-sized monitors, the polycrystalline silicon TFTs are fabricated on a glass substrate which is an insulating substrate forming an active substrate of the display device (or the so-called active-matrix substrate), because of cost limitations. In a process of fabricating TFTs on a glass substrate, processing temperatures are determined by a withstand temperature of the glass substrate.
For formation of high-quality polycrystalline silicon films (hereinafter also called polycrystalline silicon films) on a glass substrate (hereinafter also called simply a substrate), crystallization by excimer laser is utilized as disclosed in Boyce, J. B. and P. Mei: “3. Laser Crystallization for Polycrystalline Silicon Device Application,” Technology and Applications of Amorphous Silicon (2000), pp. 94-146, Springer 2000.
In a case where integrated circuits of higher performance are to be incorporated into a pixel-driving circuit, polycrystalline silicon TFTs of higher performance need to be realized. Crystallization by using solid-state laser provides polycrystalline silicon films formed of crystalline grains having their sizes extended in a scanning direction of laser and uniform crystalline grain widths in a direction perpendicular to the scanning direction of laser, and having an even surface, as described in Hara, A. et al.: “High Performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallization,” pp. 747-750, conference papers, International Electron Devices meeting (Washington D.C., 2001), and Hatano, M. et al.: “12.4L: Late-News Paper: Selectively Enlarging Laser Crystallization Technology for High and Uniform Performance Poly-Si TFTs,” pp. 158-161, SID 02 DIGEST, Society for Information Display, International Symposium Digest 2002, for example. Polycrystalline silicon TFTs formed of the thus obtained polycrystalline silicon films were reported to have improved performances of thin film transistors.